Biodiesel fuel (hereafter abbreviated as BDF), which is an alternative fuel to petroleum, is obtained by etherifying raw material fat (fat and oil) with methanol to produce fatty acid methyl esters [FAME], and mixing FAME with conventional light oil. The mixing ratio of FAME is generally about 3 to 90 volume % in the total amount of FAME and light oil. Although various animal and vegetable fats are used as the raw material fat, vegetable-derived fats, such as rapeseed oil, soybean oil, palm oil, coconut oil, and sunflower oil, are the current mainstream.
As for FAME for use in BDF, various products have been proposed in the past; however, the type and amount of BDF widely vary depending on the area where BDF is used. Therefore, fluororubbers are suitably used as rubber materials that can correspond to all of such variations.
More specifically, as a rubber composition having fuel oil resistance (sour fuel resistance), biofuel resistance (rapeseed oil resistance), and ozone resistance, and usable as an inner tube rubber of a fuel rubber hose, Patent Document 1 proposes a sulfur-vulcanizable rubber composition comprising 2 to 5 parts by weight of magnesium oxide based on 100 parts by weight of hydrogenated NBR-polyvinyl chloride blended material. The reference example of Patent Document 1 describes a fluororubber composition prepared by compounding 3 parts by weight of MgO, 6 parts by weight of Ca(OH)2, and 3 parts by weight of CaO to a non-sulfur-vulcanizable fluororubber (a product of Daikin Industries, Ltd., which is considered to be a polyol-vulcanizable fluororubber). Patent Document 1 indicates that although this fluororubber composition satisfies material performance and product performance, the cost is high.
Further, Patent Document 2 proposes a rubber molded product used for contact with engine oil, prepared by containing an amphoteric oxide (a compound generally acting as acid to base, and acting as base to acid) with a polyol-vulcanizable or peroxide-crosslinkable fluororubber. ZnO, Al2O3, and PbO, preferably ZnO, are used as amphoteric oxides. Moreover, an acid acceptor (an oxide or hydroxide of divalent metal) can be used in combination with such an amphoteric oxide. Examples 1 to 3 of Patent Document 2 describe polyol-vulcanizable fluororubber compositions comprising 6.5 parts by weight or 12 parts by weight of ZnO, 5 parts by weight of Ca(OH)2, and 3 parts by weight of MgO, based on 100 parts by weight of fluorine-containing copolymer. However, as for the peroxide-crosslinkable fluororubber, an example only compounding ZnO (amphoteric oxide) is described.
The present applicant has proposed using a ZnO acid acceptor in a peroxide-crosslinkable fluororubber composition for improving fuel oil resistance etc., (see Patent Document 3). Further, acid acceptors, such as Ca(OH)2, MgO, and ZnO, are used in a peroxide-crosslinkable fluororubber composition for reducing the coefficient of friction (see Patent Document 4).
However, when peroxide-crosslinkable fluororubbers are used for BDF applications, the addition of ZnO causes a problem of material degradation, such as rubber swelling.